Mycoplasmas are widely distributed in nature and commonly produce diseases of considerable economic impact, yet little information is available concerning mechanisms of pathogenesis and effective methods of control are unavailable. Many mycoplasmas undergo rapid variations in surface proteins that are thought to be important to disease pathogenesis. The issue of whether variations in these mycoplasmal proteins is primarily a mechanism for immune avoidance or instead a mechanism for creating cells with varied functions (e.g., tissue tropism) has not been addressed. In the murine pathogen Mycoplasma pulmonis, high-frequency phenotypic variations involving changes in the highly repetitive V-1 surface antigens affect colony morphology, the susceptibility of the organism to mycoplasma viruses, the adsorption of mycoplasmas to red blood cells, and virulence. Our laboratory has shown that the V-1 antigens are encoded by a family of genes designated vsa (variable surface antigen). Site-specific DNA inversions serve to vary vsa gene expression by recombining different genes with the vsa expression site, resulting in the phase-variable production of the Vsa proteins. Recently, we have shown that the Vsa proteins have a role in protecting the mycoplasma from killing by complement. Our long-range goals are to understand the molecular basis and pathogenic significance of phenotypic variations in mycoplasma. The specific aims of the current proposal are to study the pathogenic significance of Vsa variation. (i) The role of Vsa variation in immune avoidance will be examined by comparing the mycoplasma populations that arise in immuno-competent and immunocompromised mice (rag and inducible nitric oxide synthase-deficient mutants). (ii) The role of Vsa variation in disease chronicity will be examined by determining whether a mycoplasmal mutant that is not capable of Vsa variation has a diminished ability to sustain a long-term infection. (iii) The role of the Vsa proteins in mycoplasma-complement interactions will be examined in a series of in vitro assays. (iv) The pathogenic significance of the susceptibility of mycoplasma cells to complement will be evaluated by determining whether sensitivity to complement killing correlates with a diminished ability to colonize the mouse respiratory tract. [unreadable] [unreadable] [unreadable]